With Windows 10 you can join an organisation (=Azure Active Directory) and login with your cloud credentials.
Based on the information provided here the first account per computer that joins the organisation is a local administrator. The accounts that join after that are not.
How do I make them local administrators?
The standard group add dialog does not allow me to select users from AzureAD, search from users from AzureAD. I simply can see that my first account is in the list (listed as AzureAD\AccountName).
Interesting is also: When I login with the second account and get prompted for a local administrator (for applying computer settings - UAC I assume) it will not accept the first account even though it is a local administrator.